1. Field of the Invention
The present invention relates to an electromechanically actuable parking brake for motor vehicles, which is embodied as a drum brake, preferably of the “duo-servo” type, having a brake actuating device which can be actuated by an electromechanical actuator, is designed as an expander element and acts on two brake shoes, the expander element being supported via a spring energy storage device, which is preferably designed as a diaphragm spring assembly, and being spreadable by a predetermined length of travel, when the parking brake is actuated, to apply an application force. The invention furthermore relates to a method for actuating a parking brake of this kind.
2. Description of the Related Art
The term “drum brake” is taken to mean a brake in which brake linings interact with a cylindrical surface. When the brake is actuated, they are pressed against an encircling drum from the outside or the inside. A characteristic feature of a drum brake of the “duo servo” type is a freely movable or floating support device which lies opposite a brake actuating device in the form of an expander lock (also referred to as an expander module or expander element) and is arranged between the brake shoes. A duo-servo brake of this kind is often accommodated in the pot of the rear disk brake and is used as a parking brake, especially on motor vehicles of relatively large mass subject to exacting requirements as regards comfort.
Printed publication WO 2004/059189 A1, which is incorporated by reference, relates to an electromechanically actuable parking brake of this kind for motor vehicles, said brake being embodied as a drum brake of the “duo-servo” type. The expander element (expander lock) is formed by a threaded nut/spindle arrangement, the threaded nut of which is driven by an electromechanical actuator via a helical gear. The actuation of the threaded nut by the actuator imparts a translational motion to the spindle and brings the two brake shoes into engagement with the brake drum with the desired application force. As a result, the application force is reliably assured during a braking operation for parking, even when the brake linings are worn.
An electromechanically actuable parking brake embodied as a drum brake of the “duo-servo” type is likewise known from printed publication DE 10 2004 049 434 A1, which is incorporated by reference. The expander element of this drum brake is formed substantially by a threaded nut/spindle arrangement and two pressure pieces, one pressure piece interacting with the spindle and the other pressure piece interacting with the threaded nut. A spring element (spring energy storage device) is provided in the force transmission path between the threaded nut/spindle arrangement and the pressure piece interacting with the threaded nut. If the motor vehicle is parked on a slope, there is a slight movement of the motor vehicle in the direction of the downward force associated with the slope after the desired application force has been set. During this process, the brake drum rotates by a certain angle until the self-energizing effect characteristic of a drum brake of the “duo-servo” type takes effect. As a consequence, the desired application force is first of all reduced, this being compensated for by the spring element.
In the abovementioned electromechanically operated duo-servo brake, the expansion force does not fall below the level of the design force in any braking operation for parking. If the motor vehicle is parked on a level surface or a slope with a relatively shallow inclination, there is no change in this configuration, even though the inclination of the slope will probably be less than that for which the system has been designed in most cases. The effect of this design is that, when the parking brake is actuated, the expander element is spread apart by a length of travel that takes into account a minimum holding moment and the expansion travel that needs to be provided for the duo-servo effect, based on the design slope force. With such a design there is the problem that, in a critical case, in which the motor vehicle is parked on a level surface while still “hot”, i.e. with a hot disk brake or other heated elements of the parking brake, it is no longer possible to release the parking brake after cooling. This is due to the fact that, after the motor vehicle cools down, the brake drum, against which the shoes are resting, contracts, with the result that the application force increases and the system is shifted towards larger forces in the force/travel diagram. In the worst case, this effect can increase the application force to such an extent that it is no longer possible to disengage the brake or that brake components are destroyed.